<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>5.1什么是javascript</title>
  
  <script>
   document.write("<h3>Hello World</h3>")
  </script>
</head>
<body>
  <div onclick="alert('hello world!')">点我一下</div>
</body>
</html>